In broadcast and communication applications, trellis coding is employed to improve signal noise immunity. Trellis coding is used in combination with other techniques to protect against particular noise sources. One of these techniques is data interleaving, which is used to protect against interference bursts that may occur during transmission. In this technique, data is arranged (interleaved) in a prescribed sequence prior to transmission, and the original sequence is restored (deinterleaved) upon reception. This operation spreads or disperses the data in time in a predetermined sequence, such that a data loss during transmission does not result in a loss of contiguous data. Instead, any data lost is dispersed and is therefore more easily concealed or corrected. Another technique used to provide interference immunity is interference rejection filtering, which may be used to protect a signal against data dependent cross-talk and co-channel interference.
Trellis coding requirements for high definition television in the United States are presented in sections 4.2.4-4.2.6 (Annex D), 10.2.3.9, 10.2.3.10 and other sections of the Digital Television Standard for HDTV Transmission of Apr. 12 1995, prepared by the United States Advanced Television Systems Committee (ATSC) (hereinafter referred to as the HDTV Standard). The HDTV Standard presents a trellis coding system that employs an interleaving function involving 12 parallel trellis encoders at a transmitter and 12 parallel trellis decoders at a receiver for processing 12 interleaved datastreams. The HDTV Standard trellis coding system also employs an interference rejection filter at a receiver decoder to attenuate cross-talk and co-channel interference associated with NTSC frequencies. The rejection filter as specified by the HDTV Standard is optional and may be applied dynamically depending on the particular data being decoded.
The use of an interleaved code or dynamically selectable filter functions together with trellis decoding introduces additional trellis decoder design constraints and operating modes. These additional design constraints and operating modes significantly complicate the design and implementation of the trellis decoding function for HDTV receiver applications, for example. In particular, complications arise when the trellis decoder is required to provide seamless switching between multiple modes, such as may occur when switching between NTSC filtered and non-filtered input data or when switching between HDTV program channels, for example. In addition, cost and hardware constraints associated with consumer HDTV receivers require an efficient cost-effective trellis decoder design. Such a cost-effective design solution would employ an efficient trellis decoder architecture capable of accommodating interleaved datastreams and multiple modes of operation.